The Run-Time Library (RTL) is the core library of Delphi language. It is written in the Delphi dialect of Object Pascal and consists of a collection classes and subroutines, upon which all other Delphi applications and libraries depend. These include classes and subroutines for interacting with the underlying operating system as well has a large selection of general purpose utility classes and subroutines. A number of subroutines are inherited from Delphi's Pascal roots but much of the RTL was developed by Borland for use in its Delphi and C++ Builder software development environment and continues to be expanded upon by Embarcadero.
Much of the RTL is cross-platform but there are portions that are operating system specific. In recent releases the RTL has been split into smaller groups of functionality using unit scopes/namespaces. Now only the units under the System scope/namespace are considered part of the RTL.